1. Field of the Invention
The present invention relates to a switching network for a fast packet switching system and, more particularly, to a self-routing multistage switching network for a fast packet switching system applicable to multimedia communications in which information transmitted in different media including voice, data and image is integrated at a high speed and high throughput.
2. Description of the Prior Art
The conventional communication networks, such as a telephone network, data communication network and facsimile communication network, have been individual communication networks for one-to-one communication service.
The diversification and increase in capacity of communications has become an increasing demand of the society with the rise of the significance of information in the society. Particularly, the recent diversification of communication media brought about rising demand for multimedia communications capable of integrating information transmitted by different communication media, such as voice, data and image, to construct an economical communication network. An economical, high-quality digital communication network, such as an integrated service digital network (ISDN), has been developed to meet such a demand.
It is expected that advanced communication service systems of the next generation applicable to communication of higher speed, such as teleconference communication, fast facsimile communication and fast file transmission, will be developed in addition to the present voice or data communication service of a 64 kbit/s system.
In such an advanced communication service system, since the operating speed of the communication media is very high, it is essential that a switching system to be incorporated into such a fast communication network is capable of operating at a high operating speed and has a large capacity.
To meet such requisite conditions, studies have been made to develop a fast packet switching system incorporating a multistage switching circuit network.
The multistage switching circuit comprises a comparatively small number of hardware elements and is capable of constructing a large-scale switching circuit, and hence many studies of the multistage switching circuit have been made in the field of switching networks and parallel computers. Trials have been made to apply the multistage switching circuit to a packet switching system.
In transmitting a packet having a routing tag indicating a destination in the header section thereof by using a self-routing multistage switching system, one half of the multistage switch acts as a sorting network for permuting packets, while the other half of the multistage switch acts as a routing network. In the packet switching system, packets arrive randomly at the switch network. Therefore, the switching system must suppress internal blocking. According to one method of suppressing internal blocking, the unit switches of the switching network is provided with a buffering function, and one half of the multistage switch is provided with a sorting network to distribute the load uniformly to the unit switches. The unit switches of the sorting network select output terminals randomly to transmit packets. Consequently, the sorting network distributes load as uniformly as possible, and the unit switch of the routing network selects an output terminal and sends out packets toward a target output port. When the same output terminal is selected by two unit switches, two packets must be sent to one output terminal, and hence the unit switches must have a packet buffering function.
However, in the construction described hereinbefore, all the distributed unit switches must have a buffering function, and the distribution of buffers entails split loss, and thereby a large-scale system is necessary. A packet applied to the input port of a self-routing multistage switch is buffered by the unit switch until the output terminal of the unit switch becomes empty. Since a plurality of packets of a single communication are transferred through different routes, the packets are transferred in different transfer times. Therefore, in some cases, the packets are not sent out in the order of arrival at the self-routing multistage switch, namely, the packets contend with each other. Accordingly, in such a case, the packets arrived at the output port must be permuted in the order of application to the self-routing multistage switch to transmit a series of pieces of information correctly. However, permutation of packets requires a system having a very large scale.